Sensing a change in ambient temperature is a fundamental requirement of many electronic applications employing integrated circuits. For example, it can be essential to monitor the ambient temperature of a Central Processing Unit (CPU) in a Personal Computer (PC) so that corrective action can be taken should the ambient temperature of the CPU rise above a predetermined threshold into a temperature band where damage to, or inefficient operation of, the CPU will result.
Typical Integrated Circuit (IC) solutions for sensing temperature change use a semiconductor temperature sensor manufactured, entirely on the IC. The semiconductor temperature sensor can be a forward-biased pn junction diode or variations thereof as is known in the art, such as a diode connected npn or pnp bipolar transistor. For a forward-biased pn junction diode, the relationship between current (I) and voltage (V) (the I–V relationship) is closely approximated by the following expression:   I  =            I      s        (                  ⅇ                  V                      nV            T                              -      1        )  where:
Is is normally called the saturation current and is directly proportional to the cross-sectional area of the diode pn junction;n is the diode ideality factor (assumed hereinafter to be equal to 1), and VT is the thermal voltage, given by:       V    T    =      kT    q  where:                k=Boltzmann constant        T=temperature in degrees Kelvin        q=electronic charge        
The temperature dependence of such a forward-biased diode is well known. At a given constant current, the voltage drop across the diode decreases by around 2 mV for every 1° C. increase in temperature, owing to the dependence of Is and VT on temperature.
The known ICs exploiting the above described voltage-temperature dependence of diodes typically comprise a pair of parallel circuit branches each having a forward-biased pn junction diode, each pn junction having a different cross-sectional area and therefore a different saturation current, Is. In use, both circuit branches are held at a constant, equal current by separate biased current sources so that the voltage drops across both diodes differ. The diodes are each coupled to an output stage comprising bipolar transistors arranged so as to determine a ratio of voltage drops, each voltage drop being across each diode in each branch. Since the respective voltage drops across each of the two diodes change with ambient temperature, a voltage corresponding to the ambient temperature can be generated by the output stage, and an amplified output signal can be generated by the output stage for communication to, for example, a control circuit that controls a device for adjusting the IC temperature. This method of temperature sensing is known in the art as the kT/q method or the VT current density method.
However, such circuits suffer from the disadvantage that measurement errors of the diode voltage can result from a drawing of current from the diodes by base terminals of the bipolar transistors of the output stage. In order to mitigate such measurement errors additional current compensation circuitry must be employed together with substantial calibration of the amplified output signal generated by the output stage. Furthermore, in order to maintain an equal, constant current through each of the circuit branches in the event of, for example, fluctuations in the amplitude of the supply voltage, additional biasing circuitry must be employed as an input stage to the diodes. Such additional circuitry takes up valuable IC space and adds to the costs and complexity of the IC manufacture.